This invention relates in general to ion implantation processes and relates more particularly to an implantation process that enables the production of symmetrical MOS transistor junctions.
In the figures, each element indicated by a reference numeral will be indicated by the same reference numeral in every figure in which that element appears. The first two digits of any four digit reference numerals and the first digit of any two or three digit reference numerals indicates the first figure in which its associated element is presented.
In ion implantation processes, the depth of implantation of implanted ions increases greatly if the direction of implantation is along a major crystal symmetry direction (see, for example, Michael I. Current, et al, Planar Channeling Effects in Si(100), Nuclear Instruments and Methods in Physics Research B6(1985), 336-348). Thus, junction depth of implanted junctions can vary greatly if the implantation direction is nearly parallel to a major crystal symmetry direction. Channeling effects are commonly reduced by directing the implantation beam along a direction between 7 and 10 degrees away from the major crystal axis which is usually coincident with the perpendicular direction to the wafer top surface. The selected angle of implantation depends on the choice of ion, the implantation energy and the crystal orientation. Unfortunately, such tilt of the implantation beam away from the direction perpendicular to the top surface 19 of the wafer introduces an asymmetry into the implantation process. For example, in the production of MOS transistor junctions, this asymmetry in the implantation process produces an asymmetry between the MOS source and drain regions.
This asymmetry is illustrated in FIG. 1. In that figure, gate 11 and field oxide regions 12 and 13 have already been formed on a substrate 14 before the implantation step illustrated in this figure is commenced. Gate 11 typically consists of an oxide layer of thickness on the order of 100 Angstroms on top of which is a polysilicon layer of thickness on the order of 3,000-5,000 Angstroms. The implant ions are directed along a direction D that is at an angle A on the order of 7-10 degrees from the perpendicular N to the substrate to surface 19. In comparison to an implantation along the direction N, when the ions are implanted along the direction D, ions are blocked from being implanted in region 15 and are allowed to be implanted into region 16. Because of this asymmetry, source region 17 is not symmetric with drain region 18. Thus, the benefits of greatly reduced channeling are achieved at the cost of asymmetric junctions.
There are two common techniques for achieving symmetric junctions while avoiding the channeling problems that generally occur when the ions are implanted along the direction N. In the first of these methods, illustrated in FIGS. 2 and 3, the implantation process occurs in two steps.
In the first step, illustrated in FIG. 2, some nondopant ions are implanted along the direction N into a substrate 24 to a dept at least as great as the selected implantation depth for the dopant ion implantation step. Typically, ions of energy 5,000-180,000 ev are implanted to a depth of 2,000-10,000 Angstroms. This produces implanted regions 27 and 28 that are not only symmetric with respect to a gate 11, but are also substantially amorphous. That is, this first step has produced amorphous regions 27 and 28 that do not have the crystal pattern that results in the channeling effect. Therefore, dopant ions can be implanted along the normal direction N without experiencing the effects of channeling. Thus, in the second step, also illustrated in FIG. 2, dopant ions are implanted along the direction N. Because the ions are implanted along the direction N, the source and drain regions 27 and 28, respectively, are symmetric with respect to the gate 11. Because of the lack of crystal symmetry directions, the dopant ion concentration exhibits a sharply peaked profile as a function of implantation depth and does not include a channeling tail as would occur if regions 27 and 28 were crystalline.
In the second of the two above-mentioned methods for producing symmetric junctions without channeling effects, instead of producing deep (on the order of 10,000 Angstroms) amorphous regions in the substrate by a deep implantation of nondopant ions, only thin amorphous layers are produced at the top surface 19 of substrate 14. In a first variant of this method, illustrated in FIG. 3, thin amorphous regions, such as thermal oxide or CVD oxide, are formed on the top surface 19. As is discussed in the above-mentioned paper by Michael Current, an oxide layer on the order of or greater than 200 Angstroms is needed to substantially eliminate channeling. Dopant ions are then implanted along the normal direction N to produce symmetric source and drain regions 37 and 38, respectively.
In a second variant of this method, illustrated in FIG. 4, the thin amorphous regions 31 and 32 are produced by low energy implantation of the substrate with nondopant ions directed at an angle on the order of 7-10 degrees away from the normal N. The production of amorphous regions 31 and 32 is again followed by a high energy dopant ion implantation step along the normal direction N to produce symmetric source and drain regions 37 and 38. Because this layer is thin, the asymmetry of this process does not result in a significant asymmetry between the resulting thin amorphous regions 31 and 32.
In each of these variants of this method, the amorphous regions have a thickness T that is thick enough to scatter the incident dopant ions sufficiently that only a small fraction of these dopant ions can be along any given crystal lattice symmetry direction. This substantially reduces, but does not eliminate, the channeling problem. The thickness T should be thin enough that multiple scatterings within the amorphous regions do not so reduce the normal component of implantation ion velocity within the substrate that inordinately high implantation energies are required for conventional implantation depths. Thicknesses T on the order of 2,000-10,000 Angstroms are typical.
Unfortunately, both variants of producing the thin amorphous regions 31 and 32 require extra processing steps that significantly increase the production costs. Because different ions are used in the production of amorphous regions 31 and 32 than are used in the implantation of the dopants, the beam analyzing system (see, for example, U.S. Pat. No. 4,587,432 entitled Apparatus For Ion Implantation issued to Derek Aitkens on May. 6, 1986) used to select the ions for use in implantation into the substrate has to be tuned to select the ions utilized for the amorphization step and then needs to be retuned for the dopant implantation step to select the desired ions for the doping implantation step. If the ions for implantation are generated by a plasma ionization process, then the steps of tuning the plasma for the different implant ions can take as long as about an hour, thereby significantly increasing process time.
Gas flow rates and oven temperatures may also need to be altered between these two implantation steps. Depending on the ions utilized in each of these steps, the implantation chamber may need to be purged between these two implantation steps, thereby contributing additional time to this implantation process. Also, these amorphization steps can introduce impurities that reduce the operating efficiency of the resulting device. The ions used to produce the amorphous regions must have a high purity to avoid contaminating the substrate. This also increases the cost of manufacture.
When an oxide layer is produced on the surface 19 as amorphous regions 31 and 32, the subsequent dopant implantation step can drive some of the oxygen atoms into the substrate. This can produce defects in the source and drain regions that must be eliminated by an additional annealing step. The oxygen that is driven into the substrate can also result in the production of poor electrical contact of these source and drain regions with associated conductive contacts. Furthermore, when oxide amorphous regions 31 and 32 are formed, these regions must be stripped before contacts can be formed with the source and drain regions, thereby adding a further step to this integrated circuit manufacturing process.